Thermodynamic study on removal of magnesium from lithium chloride solutions using phosphate precipitation method

The separation of magnesium and lithium is the key issue of preparation of pure lithium product due to their similar chemical property. Thermodynamic analysis was conducted on the removal of magnesium from lithium chloride solutions with phosphate precipitation method. The thermodynamic equilibrium diagrams of Mg2+-PO43--H2O,Li+-Mg2+-PO43--H2O and Li+-Mg2+-NH4+-PO43--H2O systems at 298 K were established, separately. The effect of process parameters on the removal of magnesium was also investigated. The results revealed that the existence of Li+ was not beneficial to the removal of magnesium, and the precipitation method of MgNH4PO4 performed better than that of Mg3(PO4)2 for the separation of magnesium from lithium chloride solutions. The equilibrium concentration of magnesium decreased with the increase in total concentration of NH4+ and the decrease in concentration of Li+. With the precipitation method of MgNH4PO4, there are four regions of stable solid phase in the pH range of 4.5-13.0 in lithium chloride solutions containing 1 mol/L Li+, 0.03 mol/L NH4+, 0.01 mol/L Mg2+ and 0.03 mol/L PO43-, which are MgHPO4 (5.9 < pH < 6.6), MgNH4PO4 (6.6 < pH < 9.6), Mg(OH)2 (10.0 < pH < 13.0) and Li3PO4 (6.6 < pH < 1.3), respectively. The verification experiment results showed that the concentration of magnesium can be reduced to 0.011 g/L from a feed solution containing 14.95 g/L (∼1 mol/L) Li2O and 0.45 g/L (∼0.02 mol/L) Mg2+ by adding 0.025 mol/L (NH4)3PO4·3H2O at pH 8 and 25 °C, while the concentration of residual phosphorus in purified solution was only 0.004 g/L, which was consistent with the thermodynamic analysis.